Marshall JTM 60 filter cap problem

[NovTech]

My 1995 Marshall JTM 60 1x12" started acting up a couple weeks ago...here are the symptoms:

There is a sound when I play single notes on my guitar that sounds like there is an octave pedal and the guitar is out of tune. A very dissident sound.

I think it is the filter caps since they are placed right above the power tubes. Does this make since?

The filter caps are 550 vdc and 47uF...can I replace them with 385 vdc with 47 uF? The caps I like are JJ Electronic Low ESR 47 µF/385V Electrolytic Capacitor. Can I use these as a direct replacement?

[bluesfreak]

COuld be a microphonic tube as well, have you checked already using chopstick method?
385V are too low for the filter caps, JJ also has 47µF/500V available, if you can't find them elsewhere check with tube-town

[tubeswell]

Yep - change the tubes before anything else and report back. Its usually the simplest thing to fix and half the time its worn-out tubes which cause problems. Keep a replacement set of output tubes handy in any case, because you'll need them sooner or later, and you'll want to replace them eventually to; 1) restore the goodness in the tone; and 2) avoid any more-serious problems developing.

[NovTech]

Tubes are good...

I've tried the chopstick method, the tubes are not microphonic...I've tried replacing the tubes, same problem...

what is the lowest Vdc rating that would be safe for the filter caps? The originals are 550vdc 47uF...I can find 47uF no problem, but it is harder to find 550Vdc. I have found some at 450Vdc made by Xicon. Are Xicon filter caps ok?

Any suggestions for a good cap (brand) match for this amp (Marshall JTM 60)? It has a similar sound to the plexi...

[NovTech]

Are 450Vdc 47uF caps acceptable to use? I found them at an electronics store near my house.

[rf7]

Korg USA has them for $12.75 apiece.

[bluesfreak]

450V is borderline, can and will work but you are on the edge. Antique Electronic Supply has 47µF/500V by F&T (preferrable) and Illinois.....

[AmpFix]

450v is not enough IMO. 500v is the absolute minimum. The only way to really tell is to check your PT's B+ supply. what is it?

Panasonic makes good caps and this one should do the trick, depending on your B+.
Digi-Key - P7442-ND (Manufacturer - ECO-S2HP470BA)

[NovTech]

I'll check tonight the B+ tonight. Can I disconnect all wires to/from the tranformer then test the B+ since the amp is apart at the moment...

[NovTech]

I tried to get the B+ supply...

I left the input side attached to the PT (it read 120V) and I disconnected the output side of the PT. There was no DC reading from the output side, there was an AC reading of about 70V. I would expect there to be a DC reading...not an AC reading...

I decided to take your advice and use 47uF 500Vdc filter caps made by Ruby. They are high temp (105c) and only $6.99 ea on ebay. The only problem is that they are axial not radial leads. It is impossible to find radial lead 47uF 500Vdc electrolytic capacitors...

[AmpFix]

A PT only outputs AC given AC on its primary winding.
In saying that, 70v on the PT's B+ sounds low. Very low.

Also, if you had followed the digikey link I provided you, its for a radial lead 500v cap.

[NovTech]

Sorry, I thought that link was for something different...like spam or a personal web page...I just ordered the Panasonic Caps, I'll post reply when I get this thing running again...thanks.

[NovTech]

OK, I bought the Panasonic Filter caps (47uF 500Vdc)...installed them, but I noticed when I was soldering that the circuit board seat had come delaminated.

When I tried the amp it no longer had a distorted, out-of-tune sound, but the volume was dropping. Could this be due to the delaminated seats for the folter caps? Or poor soldering of the filter caps?

[AmpFix]

Bad solder joint could be the cause. Run a small wire from the cap leg to the other end of that trace directly. This way, you're creating a wire trace that allows you to ensure you have good contact.

Be careful not to overheat the solder pads on the PCB.

[NovTech]

I looked at the schematics and I don't see how I should hardwire them...

The caps are C111 and C112...

I can run a wire from W102 to the positive side of C112 and I can run one from R112 to the positive side of C111, but where should I run the negative side of the caps?

The schematic is found here

http://www.drtube.com/schematics/mar...00-61-02-1.gif

Any help would be appreciated...